Field of the Invention
The invention generally relates to access probe handoff, and more particularly, to access probe handoff of a mobile communication device operating in a selective retransmission mode.
Description of the Related Art
With growing demand for ubiquitous computing and networking, various wireless technologies have been developed, such as the Global System for Mobile communications (GSM) technology, General Packet Radio Service (GPRS) technology, Enhanced Data rates for Global Evolution (EDGE) technology, Wideband Code Division Multiple Access (WCDMA) technology, Code Division Multiple Access 2000 (CDMA2000) technology, High Rate Packet Data (HRPD) technology, Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) technology, Worldwide Interoperability for Microwave Access (WiMAX) technology, Long Term Evolution (LTE) technology, and Time-Division LTE (TD-LTE) technology, etc.
Taking the CDMA2000 technology for example, a Mobile Station (MS) should perform an access attempt when data transmissions and/or receptions are required. FIG. 1 is a schematic diagram illustrating an access attempt of a general MS which is not configured to operate in the selective retransmission mode. An access attempt includes N access sub-attempts, wherein each access sub-attempt is associated with a respective Base Station (BS) and includes M access probe sequences. Within each access probe sequence, the MS first performs a persistence check to see if another MS is transmitting data to the BS on the access channel. If there is not such a MS, the MS transmits an access probe with an initial transmission power. Subsequently, the MS waits for an acknowledgement for the access probe for a period of time (denoted as Tp). If no acknowledgement for the access probe is received during Tp, the MS increases the transmission power and uses the increased transmission power to retransmit the access probe. The retransmission of the access probe is repeated using incremental transmission power until an acknowledgement for the access probe is received or a maximum number of retries (denoted as n) is reached. If no acknowledgement for the access probe is received during an access probe sequence, the MS further holds for another period of time (denoted as Ts) and then initiates another access probe sequence.
However, according to the Third Generation Partnership Project 2 (3GPP2) specifications C.S0005-F and C.S0002-F, the access probe handoff is not allowed for an MS if the MS is configured to operate in a selective retransmission mode, i.e., when the selective acknowledgement (SACK) operation is applied. With this restriction, the MS has to stay in the same BS during an access attempt even if the forward link quality of the BS becomes very poor. Although the MS keeps retransmitting the access probe to the BS during the access attempt, success is unlikely due to the poor forward link quality of the BS. As a result, the MS may end up not being in service for a long time.